This is a good thing for this reason: it is better to expose muslims in another country than in the U.S. And as mayor of London, he will be in plain view. He will feel entitled—even pressured—to be an authentic muslim (i.e. radical).
The entire world will now see this radical for what he is.
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I am not sure that works....Look at Obama in the US presidency.
Obama created the famous Trump-shaped vacuum.;-)